Helical resistor

ABSTRACT

A wire wound resistance element for a potentiometer and a method of making the resistance element in which a plurality of the resistance elements are made by a batch method, and in which the resistance wire winding is firmly held in place without any loose ends. Also, the ends of the wire core of the resistance element are insulated to prevent the resistance wire from shorting across the wire core.

United States Patent 1 Rudd et a1.

[ June 3, 1975 1 HELICAL RESISTOR [75] Inventors: Floyd R. Rudd, NorthWales;

George D. MacKenzie, Maple Glen,

both of Pa.

[73] Assignee: TRW Inc., Cleveland, Ohio [22] Filed: Dec. 26, 1973 [21]Appl. No.: 428,041

Related US. Application Data [62] Division of Ser. No. 311,355, Dec. 4,1972, Pat. No.

[52] US. Cl. 338/264; 338/161; 338/270; 338/302; 338/321 [51] Int. Cl.i. H0lc U112 [58] Field of Search 338/264, 267, 270, 269, 338/302, 321,161, 315; 29/613 [56] References Cited UNITED STATES PATENTS 1,824,4479/1931 Richter 338/270 lemme 10/1933 Crouse 338/302 11/1962 Fletcher..29/613 Primary Examiner-E. A. Goldberg Attorney, Agent, or FirmJacobTrachtman [5 7 ABSTRACT A wire wound resistance element for apotentiometer and a method of making the resistance element in which aplurality of the resistance elements are made by a batch method, and inwhich the resistance wire winding is firmly held in place without anyloose ends. Also, the ends of the wire core of the resistance elementare insulated to prevent the resistance wire from shorting across thewire core.

4 Claims, 7 Drawing Figures HELICAL RESISTOR This is a division ofapplication Ser. No. 311,355, filed Dec. 4, 1972, now US. Pat. No;3,849,878 issued Nov. 26, I974.

The present invention relates to an electrical resistance element and amethod of making the same, More particularly, the presentinventionrelates to a wire wound resistance element for a potentiometer and abatch method for making the resistance elements.

The wire wound resistance elements used in potentiometers in general,comprise a core having an electrical resistance wire helically woundaround the core. One type of core used is a magnet wire, such as copper,having a coating of an insulating material thereover to insulate theresistance wire from the core wire. The resistance wire is generallycoated with a strip of an insulating material to hold the turns of theresistance wire in place on the core. These resistance elements aregenerally made by winding the resistance wire on an elongated length ofthe core. The insulating strip is then coated and cured on the woundcore. The wound core is then cut into the desired lengths for thepotentiometer elements, generally with a metal saw. This method has anumber of disadvantages. When the wound core is'cut into the individualelements, the sawing with a metal saw results in unwinding theresistance wire at the ends of the elements and a smearing ofthe-resistance wire across the ends of the metal core. The unraveled andsmeared ends of the resistance wire must be removed by hand labor usingtweezers. This slows down the operation and adds considerably to thecost of making the elements. Also, the insulating strips coated on thewound core tend to creep onto the wiper track area of the element duringthe application of the strip and could ruin the element.

It is therefore an object of the present invention to provide a novelwire wound resistance element for use in a potentiometer.

' It is another object of the present invention to provide a wire woundresistance element in which the turns of the resistance wire are securedin position on the core particularly at the ends of the core.

' It is a further object of the present invention to provide a novelmethod of making a wire wound resistance element.

It is a still further object of the present invention to provide amethod of making a wire wound resistance element without providing anyloose ends of the resistance wire or smearing the ends of the resistancewire across the ends of the core.

It is still another object of the present invention to provide a methodof making wire wound resistance element on a batch basis.

These objects are achieved by providing a resistance element in whichthe ends of a wire core are spaced inwardly from the ends of aninsulating layer on the core, a protection layer is coated around theresistance wire winding, and insulating films are coated on the ends ofthe wire core and extend over the end portions of the protection layer.The resistance elements are made by placing a plurality of elongatedresistance wire wound cores in a plastic block and cutting the blockinto a plurality of wafers each containing a plurality of the resistanceelements. All of the resistance elements in a wafer are treatedsimultaneously to space the ends of the wire cores from the ends of theinsulating layer thereon and to provide an insulating film on the endsof the wire cores. The resistance elements are then separated from theplastic of the wafers.

FIG. 1 is a side elevation view, partially sectioned, of the wire woundresistance element of the present invention.

FIG. 2 is an enlarged perspective view of a portion of an elongated wirewound and coated core from which the resistance elements of the presentinvention are made and illustrates the first step in the method of thepresent invention.

FIGS. 3 7 are perspective views illustrating the various steps of themethod of the present invention.

Referring initially to FIG. 1, the wire wound resistance element of thepresent invention is generally designated as 10. Resistance element 10comprises a magnet wire core 12, such as of a copper wire, having alayer 14 of an electrical insulating material, such as a polyimideresin, coated on the surface thereof. The insulating layer 14 extendsbeyond both ends of the core 12. An electrical resistance wire 16 ishelically wound around the insulating layer 14 along the entire lengthof the insulating layer 14. A protection layer 18 of an electricalinsulating material, such as a polyimide or epoxy resin, is coated overthe resistance wire winding 16. The protection layer 18 has a narrowopening 20 therethrough which extends longitudinally along the entirelength of the resistance element 10. The opening 20 exposes a portion ofthe resistance wire winding 16 to provide a track along which a movablecontact of a potentiometer can make contact with the resistance wirewinding. At each end of the resistance element 10, a thin film 22 of anelectrical insulating material, such as a polymide or epoxy resin, iscoated over each end of the core 12. Each of the insulating films 22extends across a respective end of the insulating layer 14 and onto thesurface of the protection layer 18.

To make the resistance element 10, according to the method of thepresent invention, one starts with an elongated length of the magnetwire core 12 having the layer 14 of the electrical insulating materialthereon. The resistance wire 16 is helically wound around the elongatedlength of the insulated core 12 along the entire length of the core.Theprotection layer 18 is then applied to the wound core, such as byspraying, painting, or dipping. The protection layer 18 is thenpartially cured by a short heating cycle. For example, a polyimide resinprotection layer can be partially cured by heating at a temperature of200C to 225C for 5 to 15 minutes.

FIG. 2 is an enlarged view showing an end portion of an elongated length24 of the resistance wire wound core with the protection layer 18thereon. The narrow longitudinal opening 20 is then formed in theprotection layer 18 along the entire length of the core so as to exposea portion of the resistance wire. This can be achieved either by buffingthe protection layer 18 with a narrow abrasive impregnated rubber beltor by a blast of bicarbonate soda powder. The elongated length 24 of thewound core is then cleaned and the protection layer 18 fully cured. Theelongated length 24 can be cleaned by ultrasonic cleaning in freon MT orwith methylene chloride or a detergent followed by ultrasonic cleaningin freon MT. The polyimide protection layer 18 can be then fully curedby heating at 200C for I to 4 hours.

A plurality of the lengths 24 of the woundcores, each typically between8 to 12 inches long, with the lengths 24 arranged in closely spaced,parallel relation are then encased in a block 26 of a plastic materialas shown in FIG. 3. The plastic material of the block 26 is one which isrelatively inexpensive and which is controllably soluble in a solventwhich does not attack the material of the lengths 24 of the wound cores.Polyester resins have been found suitable for this purpose. However,epoxy, polyurethane, silicone and thermoplastic resins as well as suchwaxes as candle wax are also usable depending on the material used forthe protection layers 18 and the insulating layers 14 of the woundcores. The material of the block 26 is also preferably filled withparticles of a mineral, such as mica, glass beads or silica. As many as100 lengths 24 of the wound cores can be included in the block 26.

As shown in FIG. 4, the block 26 is then cut completely through atuniformly spaced points along its length, along parallel planes whichare perpendicular to the longitudinal axes of the lengths 24 of thewound cores. The cuts can be made with any suitable tool, such as arotating circular diamond saw. The cuts are spaced apart a distanceequal to the desired length of the resistance elements 10, beingproduced, typically /2 to inch. Thus, the block 26 is divided into aplurality of wafers 28 with each wafer containing a plurality of lengths24' for producing the resistance elements 10.

The exposed ends of the wire cores 12, at each end surface of each wafer28, is contacted with an etchant to etch away a portion of the ends ofthe wire cores 12 as shown in FIG. 5. For wire cores of copper, asuitable etchant may be ammonium persulfate, nitric acid or ferricchloride. As shown in FIG. 5, this leaves the ends of the insulatinglayers 14 projecting beyond the ends of the wire cores 12.

The wafer 28 is washed and rinsed in water for removing residualetchant. Each wafer 28 is then iminersed in a suitable solvent for aperiod of time necessary to dissolve or soften the plastic material atthe surfaces of the wafers 28. As previously stated, the solvent is onewhich will slowly dissolve the particular plastic being used, but doesnot attack the materials of the wound cores. When the plastic is apolyester resin, methylene chloride has been found to be a satisfactorysolvent. Chlorinated solvents can be used for epoxy and silicon resins,alcohols or ketones for polyurethane, and various hydrocarbon solventsfor waxes. When the wafers 28 are removed from the solvent, they arewashed with water to remove the softened surface layer of the plasticand any of the solvent. This exposes a portion of the protection layer18 at each end of each of the lengths 24' as shown in FIG. 6. The amountof the protection layers 18 which are exposed will depend on the lengthof time that the wafers 28 are immersed in the solvent. For example,using methylene chloride as a solvent for a polyester resin, leaving thewafers 28 in the solvent for approximately ten minutes will dissolve asufficient amount of the plastic to expose approximately ten mils of theprotection layer 18 at each end of each of the lengths 24'.

As shown in FIG. 7, a thin film 22 of an insulating material is coatedon each end of each of the wire cores 12 with the film extending overthe exposed ends of the plastic is dissolved so as to separate theindividual lengths 24 in the wafers. After the plastic is completelydissolved, the lengths 24 proving the individual resistance elements 10are removed from the solvent and washed to remove the solvent. Theresistance elements 10 are then ready to be used in potentiometerassemblies.

Although the method of the present invention has been described withregard to making straight resistance elements 10, it can also be used tomake resistance elements which are in the form of a helix. For helicalresistance elements, the elongated wound cores 24 are wound in a helixand a plurality of the helically wound cores 24 are molded in a plasticblock 26. The block is then handled in the same manner as previouslydescribed.

The method of the present invention has many advantages some of whichare the following:

1. It allows large batch processing of the resistance elements so as toreduce the per unit cost of manufacturing the resistance elements.

2. It eliminates the need of peeling back by hand the loose ends of theresistance wire of each of the resistance elements, since the resistancewire is firmly held in place by the protection layer and the plasticblock when the wound core is cut into the individual elements.

3. The insulating film on each end of the wound core insulates the wirecore so as to prevent the resistance wire from shorting across the endsof the wire core, and also helps hold the ends of theresistance wire inplace.

4. The buffing operation which exposes the resistance wire path improvesthe noise characteristics of the resistance element,

5. By cutting the wafers from a large block, all of the resistanceelements in each wafer are of the same length.

Thus, the method of the present invention provides for the massproduction of the resistance elements 10 with greater ease of handlingthe resistance elements, with greater speed, at a lower cost perresistance element and with uniformity of size. Also, it provides aresistance element in which the resistance wire is firmly held in placeon the core and will not short out across the ends of the wire core.

We claim:

1. A resistance element comprising a wire core having ends and acylindrical outer surface,

a cylindrical insulating layer on the outer surface of the core withsaid layer having ends extending beyond the ends of the core which arerecessed therefrom,

a resistance wire helically wound around the insulating layer along thelength of the insulating layer and about the extending ends of theinsulating layer, and

a protection layer around the resistance wire winding.

2. A resistance element in accordance with claim 1 including aninsulating film over the ends of the wire core.

3. A resistance element comprising a wire core having ends,

, an insulating layer on the core extending beyond the ends of the core,

4. A resistance element in accordance with claim 3 in which theprotection layer has a narrow opening therethrough extending along thelength of the resistance wire winding to expose the resistance wire andprovide a track for a movable contact.

1. A resistance element comprising a wire core having ends and acylindrical outer surface, a cylindrical insulating layer on the outersurface of the core with said layer having ends extending beyond theends of the core which are recessed therefrom, a resistance wirehelically wound around the insulating layer along the length of theinsulating layer and about the extending ends of the insulating layer,and a protection layer around the resistance wire winding.
 1. Aresistance element comprising a wire core having ends and a cylindricalouter surface, a cylindrical insulating layer on the outer surface ofthe core with said layer having ends extending beyond the ends of thecore which are recessed therefrom, a resistance wire helically woundaround the insulating layer along the length of the insulating layer andabout the extending ends of the insulating layer, and a protection layeraround the resistance wire winding.
 2. A resistance element inaccordance with claim 1 including an insulating film over the ends ofthe wire core.
 3. A resistance element comprising a wire core havingends, an insulating layer on the core extending beyond the ends of thecore, a resistance wire helically wound around the insulating layeralong the length of the insulating layer, a protection layer around theresistance wire winding having an outer surface, and an insulating filmover the ends of the wire core extending over the outer surface at theends of the protection layer.